Microwave oven capacity is measured by taking the height, width and depth of the interior cavity. The three figures are multiplied together to get a total of cubic inches, then divided by 1,728 (the volume of a cubic foot). The resulting figure is the oven capacity.
For example, if the height of the microwave's cooking area is 11 inches, the width 18 inches and the depth 12 inches, then when multiplying all these together the answer is 2,376 cubic inches. That figure is divided by 1,728, with a result of 1.375. Manufacturers usually round these figures up or down. In this case, the specifications would list 1.4 cubic feet.
The calculations can be done entirely using a measuring tape and calculator. Or, the resulting figure, such as the 2,376 cubic inches above, can be plugged into an online measurement converter. One of the simplest to use is the Metric Conversion website, where users can convert the cubic inch total to cubic feet.
The outer size of a microwave can have little to do with the oven capacity. Some of the newer models have relatively thin frames and the cooking section takes up most of the machine. Others, particularly older models, are larger with thick frames, but the oven section has less volume than the smaller models.